1. FIELD OF THE INVENTION
The present invention relates to a pattern defect inspection apparatus which optically and non-contactly inspects a pattern of an integrated circuit device or the like formed on a semiconductor wafer, and more particularly to a pattern defect inspection apparatus which detects a defect in a pattern by utilizing a reflected diffraction light from a pattern.
2. BACKGROUND OF THE INVENTION
It is essential to check whether any defect is included in or any foreign material is deposited on a pattern formed on a semiconductor wafer in order to improve the yield in the manufacture of a semiconductor device.
To this end, a personal visual inspection has been performed. However, as the pattern size of the device is microminiaturized, such a visual inspection has become more and more difficult to attain, and so an effective automatic inspection has been desired.
For such demands, various methods have been proposed, such as (1) measurement of a pattern size by utilizing a laser beam or an electron beam, (2) extraction of a defect by utilizing the laser beam or the electron beam, (3) detection of foreign material by a linearly polarized laser irradiation and an optical microscope (including a detector) having deflection plates, (4) detection of a defect by comparing two picture images obtained from two image pick-up devices, such as ITV cameras (industrial television cameras) which image a pattern under inspection by two object lenses of optical microscopes, and (5) extraction of a characteristic of a pattern by analyzing the number and the directions of diffraction lights by edges of a laser beam by a plurality of light detection cells arranged cylindrically.
However, those methods are not suitable to test a defect in a pattern formed on a wafer of the type to which the present invention is directed for the following reasons.
In general, the personal inspection method, such as inspection by use of a microscope is not significantly affected by the object to be inspected. The methods (1) and (2) belong to the inspection method using a microscope. On the other hand, the methods (4) and (5) are automated forms of the personal inspection methods.
Those methods have advantages and disadvantages. The methods (1) and (2) have a disadvantage in that the inspection time is long and hence they are not suitable for use to inspect a semiconductor device in the course of a production line without taking out the semiconductor device from the line; that is, they are not applicable to a process in-line test. They are frequently utilized for a sample which transmits light, such as a photo-mask. The method (3) may be applied to a wafer which does not transmit light but it can detect only foreign material and not defects. The method (4) may be applied to a wafer, but it detects a normal area as a defect unless patterns are identical because of comparison of identical patterns. The method (5) is suitable for a photomask, but not applicable to a wafer.
As approaches to resolve the above problems, pattern defect inspection apparatus have been proposed disclosed in the assignee of the present application such as by Japanese Utility Model Application Laid-Open Nos. 55-176555 and 57-22239 and Japanese Patent Application Laid-Open No. 58-206949.
In those proposed inspection apparatus, reflected diffraction lights created by scanning a surface of an integrated circuit pattern by use of a coherent light of a predetermined light spot diameter in a normal direction with respect to the surface of the pattern is detected by light detection means arranged in a plurality of spatial areas at which the reflected diffraction lights created by a normal integrated circuit pattern does not normally reach, so that a defect in the integrated circuit pattern is detected. Various defects generated in a patterned integrated circuit or a large scale integrated circuit pattern chip (or pellet), which hardly could be detected in the past, can be optically and non-contactly detected without comparison.
In the above apparatus, however, if the pattern formed on the wafer deviates from an ideal one, any error which causes no practical problem is detected as a defect. Further, when a circular pattern having a diameter which is larger than that of the light spot is tested, it may be determined defective in spite of the fact that the circular pattern per se is not a defective pattern.
Prior to the description of the embodiments of the present invention, a diffraction phenomenon which is caused when a light beam is irradiated onto a pattern formed on a wafer is explained with reference to FIGS. 1A-1E, 2A and 2B.
FIGS. 1A to 1E show projections, on the same plane as the wafer, of reflected diffraction lights (spread into a reflection space because the wafer does not transmit a visible-band light therethrough) which are generated when the light beam is irradiated onto a strip pattern formed on the wafer.
In FIG. 1A, a direction of the edges of the strip pattern 1-a is incidental to a direction of an x-axis shown in FIG. 1E, and in FIGS. 1B, 1C and 1D, it makes an angle of 45.degree., 90.degree. and 135.degree., respectively, with the x-axis. The directions of the reflected diffraction lights generated when the light beam 2 shown by a broken line circle is irradiated onto the strip patterns make an angle of 90.degree. (right angle) to the respective pattern edges. In FIGS. 1A, 1B, 1C and 1D, 3-a, 3-b, 3-c and 3-d denote the directions of reflected diffraction lights, respectively. It is theoretically clear and experimentarily proven that the diffraction light produced by an edge is generated in the manner shown in FIG. 1.
The pattern formed on the wafer includes straight edges except for a circular pattern and an arcuate pattern and the directions thereof are limited to 45.degree., 90.degree. and 135.degree. relative to the direction of a reference pattern edge (referred to as the x-axis). FIGS. 1A to 1D show those four directions. FIG. 1E shows the directions of the reflected diffraction lights generated orthogonally to those four directions, shown in superposition. The characteristics of the reflected diffraction lights summarized in FIG. 1E are as follows. (1) The reflected diffraction lights from a normal pattern formed on the wafer regularly distribute and have predetermined directions, that is, 0.degree., 45.degree., 90.degree. and 135.degree.. Those directions are hereinafter referred to as normal directions and are designated by I, II, III and IV, respectively. (2) There are spatial areas which the reflected diffraction lights from the normal pattern do not normally reach or at which the light is very weak. Those areas are designated by A-D and A'-D'. They have four central directions, that is, 22.5.degree., 67.5.degree., 112.5.degree. and 157.5.degree.. Those are referred to as abnormal directions. (3) The intensity of the reflected diffraction light by the edge is proportional to the length of the edge irradiated by the light beam.
FIGS. 2A and 2B illustrate a defect in the pattern. In FIG. 2A, numeral 1 denotes a portion of the pattern and numeral 2 denotes a light beam having a circular cross-section.
Edges are designated by .circle.1 , .circle.2 , .circle.3 and .circle.4 . The edges .circle.1 , .circle.2 and .circle.3 are normal and the edge .circle.4 is a defective edge having a portion thereof cut away.
The light beam is reflected and diffracted normally to the edges so that the diffraction light patterns appear as shown in FIG. 2B, in which .circle.2 , .circle.2 and .circle.3 denote the diffraction light patterns by the normal edges and .circle.4 denotes the diffraction light pattern in the abnormal direction by the defective edge. Since the edge 3 is shorter than other edges, the intensity of the diffraction light is lower. This is shown in FIG. 2B by the shorter pattern .circle.3 .
As described above, the intensity of the reflected diffraction light by the edge is proportional to the length of the edge irradiated by the beam. For this reason, the reflected diffraction light by the edge .circle.3 in FIG. 2A is shown relatively short as shown by .circle.3 in FIG. 2B.
It is thus seen that in order to detect a pattern defect on the wafer, lights in abnormal directions reaching the eight areas A, B, C, D, A', B', C' and D' should be detected.
Where photo-detectors are arranged in the spatial areas A-D and A'-D', the reflected diffraction lights in the normal directions I-IV by the normal patterns are not detected by the photo-detectors but the reflected diffraction lights in the abnormal direction by the defective pattern or the foreign material are detected by the photo-detectors. This is a principle of detection of a defect.
Referring to FIGS. 3A, 3B and 4, a defect test apparatus in accordance with the above principle, which was proposed by the assignee of the present application (see Japanese Utility Model Application Laid-Open No. 57-22239) is explained, and problems associated therewith are discussed with reference to FIGS. 5A to 5F.
FIGS. 3A and 3B show spatial arrangements of detectors for detecting defects. The direction along which the areas A-A' shown in FIG. 1E extend is represented by x'. FIG. 3A shows the arrangement of the detectors in a first quadrant on a z-x' plane and FIG. 3B shows the arrangement of the detectors projected on an x-y plane (wafer surface). In FIG. 3A, numeral 4 denotes a light beam which scans a wafer 5, numerals 6-1 and 7-1 denote lenses for condensing reflected diffraction lights, numerals 6-2 and 7-2 denote lenses for directing the condensed lights to detectors 9-1 and 9-2, numerals 8-1 and 8-2 denote stray light blocking slits, numerals 9-1 and 9-2 denote the photo-detectors and numerals 10-1 and 10-2 denote pre-amplifiers. One detection system comprises 6-1, 6-2, 8-1, 9-1 and 10-1. Numeral 11 denotes a component of the reflected diffraction light from the defect (including foreign material), which is directed to the detector. This is hereinafter referred to as a signal light. Sixteen such detection systems each comprising 6-1 to 10-1, for example, are arranged as shown in FIG. 3B.
FIG. 3B shows an overall arrangement of the above-mentioned detection systems projected on the x-y plane (wafer surface). Photo-detectors 9-1 to 9-16 are shown to represent the detection systems.
As shown in FIGS. 3A and 3B, .theta. represents an angle between a z-axis and an axis of the detection system, and .phi. represents an angle between an x-axis and the axis of the detection system projected on the x-y plane. In order to detect the defects on the wafer, the detection systems each having at least one detector (two detectors in the arrangements shown in FIGS. 3A and 3B) are arranged in the eight abnormal direction areas. The construction of the detection systems is disclosed in detail in the above-referenced application (Utility Model Application Laid-Open No. 57-22239).
The outline of the inspection method of a wafer on which is formed patterns utilizing the above-mentioned detection system is as follows.
The wafer is mounted on a wafer table which is attached to an X-Y stage. A laser beam is scanned to inspect the wafer surface. The entire surface of the wafer is inspected by two-dimensionally scanning the laser beam or one-dimensionally scanning the laser beam while one-dimensionally moving the stage.
FIG. 4 is a block diagram for processing detection signals thus derived. Numeral 11 denotes the signal light, numeral 12-1 denotes a detection unit including a plurality of detection systems, numeral 13-1 denotes an analog amplifier unit including a plurality of analog amplifiers one for each of the detection systems, numeral 14 denotes a signal converter which converts as many signals as the number of detection systems into a single signal by analog-summing one half of the signals in positive polarity and the other half of the signals in negative polarity to eliminate D.C. components contained in the signals, numeral 15 denotes a full-wave rectifier, numeral 16 denotes an envelope detector, numeral 17 denotes a comparator and numeral 18 denotes a binary signal output terminal.
The defect inspection apparatus comprising the detection systems and the signal processing systems as described above has no problem in the ability of detecting a defect. However, it has the following practical problems. The characteristics (1) and (2) of the reflected diffraction lights described above are valid for an ideal pattern formed on the wafer. The ideal pattern is a pattern exactly formed as instructed by a design drawing. A lithography technique is used to form the pattern on the wafer. Because of a limitation by the lighography technique, the resulting pattern deviates from the ideal pattern. There are two deviations which are likely in the inspection of the pattern defects. One is roundness at a corner and the other is fine disturbance in the edge (hereinafter referred to as edge disturbance). Because of those two deviations, the characteristics (1) and (2) described above are not valid.
Referring to FIGS. 5A to 5F, problems encountered in detecting a defect in the actual wafer are discussed. FIGS. 5A to 5F show reflected diffraction lights for two patterns having round corners and one strip pattern having an edge disturbance. In FIG. 5A, the junction area of pattern edges .circle.1 and .circle.2 is round, in FIG. 5C, a junction area of the edges .circle.1 and .circle.2 is round, and in FIG. 5E, one side of the stripe has the edge disturbance. In FIG. 5A, when the corner area is irradiated by a light beam shown by a broken line circle 2, the reflected diffraction lights produced by the edges .circle.1 and .circle.2 are shown by .circle.1 and .circle.2 in FIG. 5B. If it is an ideal pattern, the directions of the reflected diffraction lights are limited to those two. However, weak lights .circle.a , .circle.i and (ii) are generated at the round corner area. The light .circle.1 is in the normal direction (45.degree.) and does not raise a problem in detecting the defect but the lights (i) and (ii) are in the abnormal directions (22.5.degree. and 67.5.degree.) and they are detected by the detection systems arranged in those directions so that the round corner is determined to be a defect. The above description is also applicable to the patterns shown in FIGS. 5C and 5E, and the round corner and the edge disturbance are determined as the defects.
Next, the case of the corner shown in FIG. 5C will be explained. If it is an ideal pattern, the reflected diffraction lights will be only in the normal directions .circle.1 and .circle.2 shown in FIG. 5D. However, because of the round corner, weak lights .circle.a and .circle.b in the normal directions and weak lights (i) to (iii) in the abnormal directions are generated and the lights in the abnormal directions are detected so that a defect is indicated.
In the strip pattern of FIG. 5E, if it is an ideal pattern having no edge disturbance, the reflected diffraction lights will be only in the normal directions .circle.1 and .circle.1' as shown in FIG. 5F. However, because the edge disturbance, light .circle.a in the normal direction and lights (i) and (ii) in the abnormal direction are generated and the lights in the abnormal directions are detected so that a defect is indicated. The condition of FIG. 5F varies in different ways depending on the edge disturbance.
The above deviations from the ideal pattern are inherent to the process and they are not defects from the standpoint of the device function. If such deviations from the ideal pattern are detected, even a pattern which causes no practical problem is determined as a defective pattern.
To summarize the prior art problems, the prior art wafer inspection apparatus can inspect the wafer having a light-nontransmissible pattern formed thereon and has a sufficient detection ability, but since the pattern formed on the wafer deviates from the ideal pattern, the apparatus identify the deviated areas as defects.